The present invention relates to an intensified charge-coupled image sensor and particularly to a header assembly for such a sensor that utilizes a plurality of low cost metal parts which securely retain and locate a charge-coupled device (CCD) relative to the optical axis of the sensor.
An intensified charge-coupled image sensor comprises an image intensifier section having a photoemissive cathode on an interior surface of an input window and a header assembly having a charge-coupled device (CCD) located at the focal plane of the image sensor. Such a structure is shown in U.S. Pat. No. 4,355,229 issued to H. S. Zimmerman et al., on Oct. 19, 1982, and assigned to the assignee of the present invention. The Zimmerman et al. patent is incorporated by reference herein for the purpose of disclosure. The CCD described in the Zimmerman et al. patent, and shown herein in FIGS. 1 and 2, includes an image array 10, known as an A register, a temporary storage array 12, known as a B register, and an output register 14, known as a C register. The B and C registers are usually masked, that is, mean (not shown) are provided for preventing photoelectrons from the photoemissive cathode from reaching either register.
The A and B registers have channel stops (not shown) extending in the column direction to isolate the channels (the columns of the CCD) from one another. The electrodes (shown in FIG. 2) may be of the single layer type comprising, for example, N+ type regions of polysilicon separated by P-type regions of polysilicon. These electrodes extend in the row direction and, in the example illustrated, are three-phase operated. The electrodes are insulated from the relatively thick P-type substrate by a layer of silicon dioxide (SiO.sub.2). The CCD mentioned above has 320 columns and 512 rows (256 in the A register and 256 in the B register), each row comprising a group of three electrodes.
The operation of the CCD of FIG. 1 is well understood. During the so-called integration time, a scene or other image is projected directly onto the A register. However, in an intensified charge-coupled image sensor, such as that shown in FIG. 3, the scene is projected onto the photoemissive cathode and photoelectrons released therefrom in a pattern corresponding to the intensity of the radiation incident on the cathode impinge upon the A register of the CCD. The incident photoelectrons cause charges to be produced at the various locations of the A register in accordance with the energy density reaching the respective locations.
Upon the completion of the integration time (e.g., during the vertical blanking interval of commercial television), the charge signals which have accumulated (a "field") are transferred, in parallel, in the column direction from the A to the B register by the application of the multiple phase voltages .phi..sub.A1 . . . .phi..sub.A3 and .phi..sub.B1 . . . .phi..sub.B3. The charges subsequently are transferred a row at a time, from the B register to the C register, and after each row of charges reaches the C register, it is serially shifted out of the C register in response to the shift voltages .phi..sub.C1 . . . .phi..sub.C3. The transfer of charges from the B to the C register occurs during a relatively short time (the horizontal blanking time of commercial television, which is about 10 .mu.s) and the serial shifting of the C register occurs at relatively high speed (during the horziontal line display time of commerical television. During the transfer of a field from the B to the C register, a new field may be integrated in the A register.
When a CCD is used in an intensified charge-coupled image sensor, it is necessary to thin the substrate, at least in the A register to a thickness of about 10.mu. (microns) to minimize lateral dispersion of the charge produced by the incident photoelectrons. One method for thinning a single large wafer containing a large number of CCD's is disclosed in U.S. Pat. No. 4,266,334 issued to T. W. Edwards et al. on May 12, 1981, and incorporated by reference herein for the purpose of disclosure. The Edwards et al. patent utilizes a glass plate which is laminated or glued onto the thinned surface of the wafer to add structural support during testing and removal of the individual CCD's from the large wafer. The glass support plate must, however, be delaminated or otherwise removed from the wafer before the CCD's contained therein can be used in an intensified charge coupled image sensor. The glue used to affix the glass plate to the CCD is incompatible with the formation of the photoemissive cathode formed on the interior surface of the input window of the image intensifier section of the image sensor, and the glass window also attenuates the photoelectrons from the cathode thereby preventing them from impinging upon the A register of the CCD.
U.S. Pat. No. 4,465,549 issued to I. G. Ritzman on Aug. 14, 1984, and assigned to the assignee of the present invention, discloses a method of removing the glass support plate from a thinned wafer containing a large number of CCD's. The wafer is subsequently sectioned to provide the individual CCD's. Each of the thinned CCD's has a thickness of about 10u and is too delicate to handle without damage. Thus, copending U.S. patent application Ser. No. 494,288 filed by J. A. Zollman et al. on May 13, 1983, discloses a support frame which is brazed to one surface of a thinned CCD to provide the required rigidity for handling and mounting the CCD within the image sensor. In prior art intensified charge-coupled image sensors, the CCD with the attached frame is brazed to an insulative header. The initial brazing of the frame to the CCD and the subsequent brazing of the frame with the attached CCD to the insulative header has resulted, occasionally, in scrap due to the flow of braze material onto the CCD and to misalignment of the CCD on the insulative header. Alternatively, the CCD is attached to the header by means of spring clips. However, spring clips permit some movement and misalignment of the CCD under severe shock and vibration of the sensor. Accordingly, the need exists for a reliable structure for securing and locating the CCD upon the insulative header without brazing, and for accurately centering the CCD relative to the optical axis of the image sensor.